How does it confirm she's a Lesbian? She could be bisexual.
Today, we almost have the confirmation that Seven is bisexual, especially with the heavily implied relationship between her and Bjayzl but I remind you that during Seven's journey on Voyager, we have only seen her surrounded by potential male mates (Chakotay being the only one who we have seen on screen in intimate scenes..., well if the exchange of chaste kisses can be associated with any intimacy) BUT we don't know if she ever had gone further with him, in having sex).
For my part, I remember very well, from s4, to have wondered if someone who just came out from her cocoon, like Seven with her Borg chamber, bound to be straight... . Indeed, it would have seemed much more likely to me if that, in her quest for regaining her humanity (yes, she was born as human but kidnapped and reprogrammed by Borg at 6 or 7 years so, of course, with still no real knowledge about her sexual preferences), a better knowledge of human beings and her insatiable curiosity, she wouldn't try to be intimate with both sexes, whetever human or alien then decide afterwards, which was the most enjoyable and which machted her the best, especially since she was surrounded by strong personalities as Kathryn Janeway and B'Elena Torres, who were women ; Tuvok, the Doctor who, though a hologram, has taken a male appearance (note that I didn't quote Chakotay because they rarely met from s4 to s6and each time, it was very tense between them.
And if I have written that Seven of Nine was suposed to be lesbian, it was because Bryan Fuller, who was gay - and still is, besides - wanted that this new character be lesbian. Brannon Braga, who created the character, agreed to the idea as Rick Brennan, who supported it too but the big guys of CBS/Paramount refused (surely because they have been feared of the reaction of male viewers, who were in majority, at the idea of introducing a sexy female character who would be only interested in women. Hey, they were already having trouble accepting the idea of female Captain so... .
Jeri Ryan confirmed in one of the articles mentionned in my last posts, that she would have seen no problem playing a Borg in quest of her humanity incidentally lesbian and even Kate Mulgrew wasn't opposite to see Janeway flirting with Seven like she flirted with Tom Paris and/or Chakotay (besides, in some articles, Ryan & Mulgrew admitted openly having played some of their scenes sometimes sexier than it was expected).